Mobile communication is becoming increasingly popular, at much the same time that more and more customers are asking for ever higher data-rate services. Increasingly, development efforts are focusing on techniques for high-capacity communication of digital information over wireless links.
Spread-spectrum is a method of modulation, like FM, that spreads a data signal for transmission over a bandwidth, which substantially exceeds the data transfer rate. Direct sequence spread-spectrum involves modulating a data signal with a pseudo-random chip sequence. The spread-spectrum signal is transmitted as a radio wave over a communications media to the receiver. The receiver despreads the signal to recover the information data. For a given bandwidth, processing gain and power level, a spread-spectrum communications system has a limited capacity for communicating information over a single channel.
A high processing gain means that for the information signal there is a high correlation at the receiver, creating a high signal to interference ratio. A low processing gain makes spread-spectrum communication more susceptible to interference such as from transmissions in neighboring cells.
A variant of direct sequence spread-spectrum communication involves mapping distinct information data streams to unique chip sequences. One technique for overcoming the problems relating to processing gain is disclosed in U.S. Pat. No. 5,862,133, entitled, PACKET-SWITCHED SPREAD-SPECTRUM SYSTEM, by D. L. Schilling. In the system disclosed in the '133 patent, a data stream input at a transmitter is demultiplexed into a plurality of sub-channel data-sequence signals. Each sub-channel data-sequence signal is spread-spectrum processed into a spread-spectrum signal. The spread-spectrum signals are combined and sent over a communications channel. At the receiver, the received signal is despread into the plurality of sub-channel data-sequence signals and multiplexed back to a stream of received data.
Consider the transmission of 56 Mbps data over 100 MHz wide radio frequency (RF) band, by way of an example. Processing gain at the spread level is the chip rate divided by the code rate before spreading. If data is demultiplexed onto I- and Q-channels and spread, then the processing gain at the demultiplexed channel level is around 5.5 dB. The typical advantages of spread-spectrum modulation are resistance to fading caused by multipath, immunity from inter-cell interference from spread-spectrum signals of neighboring systems. The low processing gains, however, result in channel degradation and reduce or eliminate the advantages normally associated with spread-spectrum modulation.
It may be helpful to consider an example of such a system in somewhat more detail, such as the system shown in FIG. 7. The illustrated system includes a transmitter communicating with a receiver via an air-link. The transmitter essentially includes the elements 71-75 shown in the drawing.
In the transmitter, the demultiplexer (DeMux) 71 receives the input data stream, for example at the 56 Mbps rate. The demultiplexer 71 essentially splits the data into two branches, one for the I-channel and one for the Q-channel. Here we are assuming four-bit wide sub-channel streams, therefore the demultiplexer 71 alternately sends four-bits to its I output and four-bits to its Q output, resulting in two separate 28 Mbps data sub-channel sequences.
Each sub-channel data sequence goes to an input of one of two code mapper circuits 72. Each code mapper 72 maps each four bits on its input to a distinct one of sixteen available code-spreading sequences. Each code sequence is sixteen chips long. Each mapper uses the same set of sixteen spreading codes.
A modulator 73 receives the code-spread output of the I-channel mapper. The modulator 73 multiplies the direct sequence spread spectrum by an RF oscillator signal cos(.omega..sub.0 t) or carrier wave. Similarly, a modulator 74 receives the code-spread output of the Q-channel mapper. The modulator 73 multiplies the direct sequence spread spectrum by an RF oscillator signal sin(.omega..sub.0 t). The two resultant modulated signals have the same frequency (.omega..sub.0) but have a 90.degree. phase difference. A summer 75 combines the two modulated RF signals from the modulators 73 and 74, and the combined signal is transmitted over the channel.
The channel is subject to a variety of different types of noise signals and interference effects. Theoretically, the transmission through the channel may be viewed as a summation 76 adding a noise signal n(t) to the broadcast spread spectrum signal from the summer 75.
The receiver essentially comprises the elements 77-82 shown to the right in FIG. 7. The noise corrupted signal from the channel 76 is applied to two multipliers 77, 78. The multiplier 77 multiplies the received signal by a local oscillator signal cos(.omega..sub.0 t) to translate to a desired baseband. Similarly, the multiplier 74 multiplies the noise corrupted signal from the channel by a local oscillator signal sin(.omega..sub.0 t) to translate to the baseband. The local oscillator signals essentially correspond to the signals used to modulate the two spread spectrum sub-channel sequences at the transmitter.
Each multiplier 77, 78 essentially outputs a signal containing the spreading codes plus the channel noise. Each of these output signals is applied to a matched filter (MF) 79. A matched filter includes reference signals, in this case corresponding to the sixteen spreading codes, and correlates the signal on its input to identify the most likely match (largest correlation value) and thereby selected the most probably transmitted code sequence. Each demapper 81 therefore outputs a bit stream corresponding to chip sequences recovered from the received carrier-modulated spread-spectrum signals.
Each matched-filter (MF) 79 supplies the recovered stream of chip sequences to a demapper 81. The demapper 81 maps each recovered chip sequence back to a four-bit data value. The output of one of the demappers 81 essentially corresponds to one of the sub-channel data streams at 28 Mbps. The two sub-channel data streams are input to a multiplexer (Mux) 82, which combines alternate four-bit chunks of data to form the output data stream at 56 Mbps.
While this type of spread-spectrum communication using sub-channels does achieve some improvement in processing gain at the demultiplexed sub-channel data sequence level, the improvement is somewhat limited by the available bandwidth. Absent more bandwidth, it is not possible to further increase processing gains with existing techniques. A need therefore exists for a technique achieving still higher processing gains, particularly within a given bandwidth, to allow interference-free transport of higher quantities of data over a given wireless channel bandwidth.